Spark plug



Sept 23, 1969 M, A. BRI-:TscH 3,468,004

SPARK PLUG original med July 1s, 1961 @ffl-9@ ATTORNEYS United StatesPatent C 3,468,004 SPARK PLUG Michael A. Bretsch, Toledo, Ohio, assignerto Champion Spark Plug Company, Toledo, Ohio, a corporation of DelawareOriginal application July 13, 1961, Ser. No. 123,878, now Patent No.3,256,457, dated June 14, 1966. Divided and this application Nov. 16,1965, Ser. No. 508,111

Int. Cl. H01t 13/00 U.S. Cl. 29-25.12 2 Claims ABSTRACT F THE DISCLOSUREA method for manufacturing an improved `center electrode for spark plugsused in high performance internal combustion engines. A hollow, splitsleeve of a metal having a low thermal conductivity and a low coecientof expansion is compressed and inserted into the nose bore of a sparkplug insulator. A corrosion resistant spark tip is inserted into the endof the sleeve and a metal having a high thermal conductivity and acoeficient of expansion greater than that of the metal of the sleeve iscast into the sleeve. Upon cooling, the electrode contracts in the nosebore to provide a cylindrical air gap of not more than 0.001 inch.

Cross-reference to related applications This is a division ofapplication Ser. No. 123,878, filed July 13, 1961, now Patent 3,256,457,issued June 14, 1966.

This invention relates broadly to spark plugs, and, more particularly,to ceramic insulated spark plug assemblies highly resistant to insulatorcracking caused by thermal gradients brought about during normal serviceuse thereof, and to methods of producing such assemblies.

In the use of ceramic insulated spark plugs for high output internalcombustion engines, including aircraft engines, substantial difficultyhas been encountered because the insulators have had a tendency to crackat the nose portion. The nose portion is that part of the spark pluginsulator surrounding the center electrode that extends inwardly towardsthe tiring end from the seat on which the insulator rests when the sparkplug is assembled. When the nose portion cracks the resultingdiscontinuity often causes a drastic temperature rise, with the resultthat the tip of the spark plug may run so hot as to `cause pre-ignitionin the associated cylinder and a possible failure of the engine.Pre-ignition is a serious difficulty in a high output internalcombustion engine. If such an engine continues operation in preignitionfor more than a few seconds, the extreme heat generated will damage thespark plug, and the engine. If pre-ignition does not occur afterformation of a crack in the nose portion of a spark plug insulator, thecrack may become a ycomplete fracture and a part of the insulator may bedislodged. The dislodged part may itself cause severe mechanical damagein the combustion chamber.

In addition to spark plug failure caused by pre-ignition resulting fromnose cracking as described above, preignition with resulting spark plugfailure often results when the engine output surpasses that theparticular spark plug can withstand. When a spark plug is operated in ahigh output engine, and the output of that ICC engine is graduallyincreased, an engine output is reached at which that particular sparkplug causes pre-ignition. The indicated mean effective pressure of anengine at which a spark plug, when operating Within the engine, willcause pre-ignition, is commonly denominated the I.M.E.P. rating of thatspark plug. It has been found that the I.M.E.P. rating of a spark plugcan be increased by shortening the nose thereof, but that such expedientis relatively undesirable because such plugs are particularly subject tocarbon fouling when operated at low output. It has also been found thatthe I.M.E.P. rating of a spark plug can be effectively increased withoutdeleteriously affecting the susceptibility of the plug to fouling bycasting a silver electrode part in a bore through the nose portion ofthe spark plug insulator. Such expedient, however, increases thesusceptibility of the insulator to nose cracking and, therefore, is notdesirable. As a consequence, an acute problem exists in previously knownspark plug designs: that of iinding a spark plug having a high I.M.E.P.rating which is not subject to substantial carbon fouling, while at thesame time is capable of operation in high output engines Without atendency to crack at the nose portion.

It has now been discovered, and the instant invention is based upon suchdiscovery, that the advantages of the cast in place, silver electrodestructure, can be achieved, and the tendency for the cast silver tocause nose cracking substantially eliminated or at least minimized bycasting a metal having a high thermal conductivity, for example silver,into a metallic sleeve or tube having a relatively low thermalconductivity in comparison with the cast metal, such sleeve or tubebeing disposed in at least a part of the bore through the nose of theinsulator. In a specic instance, excellent results have been achievedwhere a split nickel sleeve, before casting of the silver, has beenexpanded against the wall of the insulator bore; in such case, aftercasting of the silver or other high thermal conductivity metal, andbecause of the contraction of such metal during cooling thereof from amolten condition, an air gap is established between the wall of theinsulator bore and the metallic sleeve, with the result that the sleevetits loosely within the insulator bore, and, in fact, may even slidetherein when the assembly is tipped. It has been found that, in service,the nickel tube or sleeve, constituting a material having a low thermalconductivity relative to that of the cast silver body, and the gasfilled gap between the exterior of the tube and the insulator nose borewall, constitute thermal barriers to the transfer of heat between thenose of the insulator and the electrode part, whereby as a consequenceof the thermal barriers, stressing of the nose bore wall by the castsilver body to an extent suicient to cause insulator cracking isprevented, but, at the same time, heat transfer between the two issuiciently high that the spark plug has a high I.M.E.P. rating.

It is, therefore, an object of the invention to provide an improvedspark plug assembly.

It is a further object of the invention to provide such an assemblywhich will operate satisfactorily in engines running under a high load,or at a high I.M.E.P. Without insulator nose cracking.

.More particularly, it is a further object of the invention to provide aspark plug assembly which includes a body of a material having a highthermal conductivity, at least part of which body is within a bore ofthe insulator, and means disposed within an annular space between thebody and the wall of the insulator defining the nose bore, which meansis effective to decrease the rate of heat transfer between the body andthe nose bore wall.

It is a still further object of the invention to provide a method forproducing such a spark plug assembly.

Other objects and advantages will in part be apparent and will in partappear hereinafter.

For a better understanding of the nature and objects of the invention,reference should be had to the following detailed description andattached drawing, illustrating a preferred embodiment thereof, in which:

FIG. 1 s a vertical sectional view of a spark plug assembly embodyingthe present invention; and

FIG. 2 is a horizontal sectional view taken along the line 2 2 of FIG.1.

According to the invention, an improved spark plug assembly is provided.Such assembly, as shown in the drawings, comprises an appropriateceramic insulator indicated generally at 11. The insulator is engaged inand supported by a metal shell indicated generally at 12 which isthreaded at 13 for engagement in the combustion chamber of an engine.The insulator 11 has a nose portion 14 extending from its firing end toan intermediate area indicated generally at 15 which is operativelyassociated with the metal shell 12 for conductive heat transfer betweenthe two. The insulator 11 has a central bore indicated generally at -16and formed in steps as indicated at 17 and 18. The step 18 is providedin the bore through an intermediate insulator portion 19 so that thebore diameter defined by a nose bore wall 20 is less adjacent the firingend of the insulator than through the intermediate portion. A splitmetallic tube or sleeve 21, which in the specific embodiment is nickel,is disposed in the smaller diameter insulator bore portion, adjacent theinsulator firing end, between the nose bore wall 20 and an electrodepart indicated generally at 22. The electrode part 22 includes a firingtip 23, the lower extremity of which constitutes a sparking surfaceoperatively associated with a ground electrode 24 which is structurallyintegral with the metal shell 12. The electrode part 22 also includes abody 25 of cast metal, which in the particular embodiment is silver. Thebody 25 is in both mechanical and electrical contact with the firing tip23, and also with an upper electrode part 26. The upper electrode part26 extends upwardly through a suitable gas-tight seal 27 and makeselectrical contact with a resistor part 28 which is urged thereagainstby a spring 29 that is confined within a cap member 30. The nickelsleeve 21 is positioned in closely adjacent, spaced relationshiprelative to the wall of the insulator defining the nose bore. Foroptimum plug performance the spacing is kept to a minimum, andpreferably is not greater than about 0.001. Excellent results have beenachieved when this spacing has been approximately 0.0005". This spaceconstitutes a gas-filled gap between the exterior of the sleeve and theinsulator nose bore wall 20.

The assembly illustrated in the drawing can be readily produced byinserting the split nickel tube or sleeve 21 into the bore of theinsulator in the position shown in FIG. 1. The sleeve 21 preferably hasan exterior diameter slightly greater than the interior diameter of thebore in which it is inserted so that it is compressed slightly when inposition. After insertion, the sleeve is turned outward at its upper endto conform to the stepped insulator bore at this point, and to form amechanical lock preventing the downward movement of the sleeve. rlihefiring tip 23, preferably made of platinum or other corrosion resistantmetal, is then dropped into the sleeve with one end protruding throughthe hole in the sleeve bottom. A rod of silver is next positioned abovethe firing tip with a washer 31 and the upper electrode part 26positioned thereabove. The resulting assembly is then placed in asuitable furnace and heated to a temperature suiciently high to fuse thesilver rod (melting point 960 C.), and thereafter cooled sufficientlyslowly to prevent cracking. The silver, upon fusion, in effect for-ms abrazing-type of bond with the nickel sleeve so as to become integraltherewith, no substantial alloying, however, taking place between themetals. Upon cooling, the silver, of course, contracts, and pulls thesplit nickel sleeve into a smaller diameter whereby the air filled gapis formed between the outer diameter of the sleeve and the insulatornose bore wall. The gas-tight seal 27 is then formed in any suitableway; the insulator is inserted in the metal shell 12, with suitablegasketing, and the shell is crimped into engaging and supportingrelationship relative to the insulator; and the remainder of theassembly is completed in a conventional manner.

It will be apparent from the above description that a metallic sleeve,having a lower thermal conductivity than the cast-in-place electrodepart, is disposed between the cast metallic body and the wall of theinsulator which defines the bore through the nose thereof, or the nosebore wall. It will be appreciated that the sleeve or tube material, byvirtue of the difference in thermal conductivity, constitutes a thermalbarrier between the cast electrode part and the ceramic insulator. Inaddition to the sleeve or tube itself serving as a thermal barrierbetween the cast electrode part and the ceramic insulator, the air gap,formed by contraction of the silver as described, serves as a furtherbarrier to the transmission of heat between the cast electrode part andthe insulator. However, neither of the thermal barriers above describedis of sufficient magnitude to lower the I.M.E.P. rating appreciablybelow that obtained with a conventional cast silver electrode assemblywithout such barriers. In other words, even though the thermal barrierresulting from the sleeve and air gap in spark plug assemblies producedin accordance with the invention is effective substantially to preventinsulator nose cracking, still the sleeve material has a sufficientlyhigh thermal conductivity that heat transfer -between the electrode andinsulator is high, and the spark plug assembly has a high I.M.E.P.rating.

It has been found that substantially equivalent results can be achievedby what may be denominated a reversal of the air gap relative to thesplit sleeve. For example, the interior of the sleeve can rst beoxidized to prevent adhesion of silver or the like thereto, and can thenbe positioned in the insulator bore as described, with a suitablebonding material disposed between its exterior wall and the insulatorbore. This bonding material can be silver, or any suitable ceramicbonding material. In this instance, when the assembly is heated, withsilver in position to fiow into the split nickel sleeve upon melting,the sleeve is bonded to the insulator bore but the oxide coating on theinterior of the tube prevents adhesion with the silver therein, with theresult that contraction upon cooling provides an air gap to act as athermal barrier between the body of silver and the nickel sleeve. Bycarrying out the procedure which has been described in this paragraphexcept that the silver or other bonding material is omitted from betweenthe exterior of the nickel sleeve and the insulator bore, a stillfurther modified result can -be achieved: a limited thermal barrierbetween the exterior of the nickel sleeve and the insulator bore and agreater thermal barrier between the silver body and the interior of thenickel sleeve.

The sleeve or tube is also effective to prevent direct contact betweenthe cast body and the nose bore wall, and, therefore, when the assemblyis heated in service, prevents stressing to an extent which would besufficient to cause insulator cracking, of the nose bore wall by theexpanded cast body.

As will be readily appreciated, the sleeve material, in addition tohaving a suitable thermal conductivity, must possess a certainrefractory or mold-like property which enables a metal having a highthermal conductivity and forming the electrode part, for example,silver, to be cast therein without causing either a chemical or physicalchange in the structure of the sleeve material. For example, the sleevematerial must have a considerably higher melting point than the castmetal, and must be inert with respect to such cast metal at thetemperatures employed for casting so that substantially no interfacealloying, which would tend to materially lower the thermal conductivityof the cast electrode part, takes place. As previously indicated nickelhas been found to be especially well suited for use as the sleevematerial. In addition to nickel, other specific materials that may bementioned as particularly well adapted for use as the sleeve materialare certain nickel alloys such as D Nickel (95 percent nickel, 4.5percent manganese) and Inconel (76 percent nickel, 13 percent chromium,7 percent iron) and other minor constituents, as well as chromium,molybdenum, and iron. In general, high temperature metals and alloyshaving a thermal conductivity not greater than 1A c.g.s. are preferred.Thermal conductivity as used herein and in the appended claims, isdefined as the quantity of heat in calories which, at room temperatureor approximately 18 C. to 22 C., is transmitted per second through aplate one centimeter thick across an area of one square centimeter whenthe temperature difference is 1 C.

As has hereinbefore been indicated, the cast material must be one thatpossesses an extremely high thermal conductivity. In addition to silver,excellent results may be obtained with aluminum, copper, and gold,employed as the cast electrode part. In this respect, it has been foundthat the cast material must have a thermal conductivity of at least 0.5.Therefore, any metal or alloy capable of being cast under normalconditions, and having a thermal conductivity, as measured in the abovemanner, of at least 0.5 is suitable for use as the cast electrodematerial in the present invention.

It will be appreciated that an insulator can stand a certain magnitudeof thermal stress in service without cracking, but that the magnitude ofthermal stress which causes cracking depends upon the composition of theinsulator. For example, certain insulators containing large proportionsof BeO are capable of withstanding much higher thermal stresses, withoutcracking, than are more conventional insulators containing at least 90percent of A1203. High alumina insulators, however, are considerablymore resistant to cracking when subjected to thermal stress than arestill other insulators that have been used or might be used. In itsessential details, the instant invention contemplates a thermal barrier(the sleeve or tube and air gap) disposed between a body of a materialhaving a high thermal conductivity (for example, silver) and aninsulator nose bore wall. Ideally, the magnitude of the thermal barriershould be just sufficient to prevent cracking of the particularinsulator by thermal stress encountered in service. An assembly,including such athermal barrier, has a maximum I.M.E.P. rating, for agiven design, for the particular insulator material. However, a lesserthermal barrier would suffice to prevent nose cracking of an insulatorcapable of withstanding a high thermal stress, and a greater thermalbarrier would be required to prevent nose cracking of an insulator whichcould withstand only a smaller stress. Accordingly, an assemblyaccording to the invention must include a thermal barrier of sufiicientmagnitude to prevent nose cracking, but the magnitude thereof cannot bedefined numerically because it depends upon other factors.

It will be apparent from the foregoing description of the method forproducing a spark plug assembly according to the invention that a newand useful method improvement as Well as a new and useful spark plugassembly is provided. Such method is for producing a spark plug assemblycomprising an insulator that is to be engaged and supported by a metalshell, the insulator having a terminal portion and a nose including afiring end, the nose extending from the firing end to an intermediateinsulator area adapted for operative association with the shell forconductive heat transfer between the shell and the sleeve or tube, thenose having a bore defined by a nose bore Wall, and a terminal portionhaving a bore axially aligned with the nose bore.

The method includes the steps of forming at least a portion of anelectrode part by casting a metal or metal alloy having a thermalconductivity of at least 0.5 within the insulator nose bore, andadditionally supporting in the insulator nose bore, prior to the castingstep, a split metallic sleeve or tube of a material having a relativelylow thermal conductivity in comparison with the cast electrode part,which sleeve or tube is effective substantially to prevent directcontact between the insulator nose bore wall and the cast material, bothduring casting and subsequently, and which is also effective to spacethe cast body from the nose bore wall a distance sufficient that theinsulator does not crack under service conditions when in the combustionchamber of an internal combustion engine operated from an idle conditionto a high-load condition at which pre-ignition occurs. The spacingprovided should also be sufficiently small that the cast body iseffective to increase the I.M.E.P. rating of a spark plug assembled fromthe insulator structure.

While it is believed that the more advantageous embodiments of theinvention have been described, it is apparent that many modificationsand variations can be made in the specific construction, arrangement, orform of the parts, and in specific procedure discussed without departingfrom the spirit and scope of the present invention, as those skilled inthe art will readily understand. Such modifications and variations areconsidered to be within the purview and scope of the invention asdefined by the appended claims.

What I claim is:

1. In a method for producing an assembly comprising a spark pluginsulator and an electrode part, the insulator being adapted to beengaged and supported by a metal shell, and having a terminal portionand a nose including a firing end, the nose extending from the firingend to an intermediate insulator area adapted for operative associationwith the shell for conductive heat transfer between the two, the nosehaving a bore defined by a nose bore Wall, and a terminal portion havinga bore axially aligned with the nose bore, the improvement comprisingthe steps of providing a split metallic sleeve consisting of a metalhaving a predetermined low thermal conductivity and predetermined lowcoeicient of expansion within said insulator nose bore in contact withsaid insulator nose bore wall, and casting within said metallic sleeve amolten material having a thermal conductivity of at least 0.5 and acoefficient of expansion higher than that of said metallic sleeve,whereby said sleeve and said cast material contract upon cooling in saidnose bore to provide a cylindrical air gap of not more than about 0.001inch.

2. In a method for producing an assembly comprising a spark pluginsulator and an electrode part, the insulator being adapted to beengaged and supported by a metal shell, and having a terminal portionand a nose including a firing end, the nose extending from the firingend to an intermediate insulator area adapted for operative associationwith the shell for conductive heat transfer between the two, the nosehaving a bore defined by a nose bore wall, and a terminal portion havinga bore axially aligned with the nose bore, the improvement comprisingthe steps of providing a split metallic sleeve in contact with saidinsulator nose bore wall, said sleeve having a predetermined lowcoefficient of expansion and a thermal conductivity of less than 0.5,and casting within the split metallic sleeve a material which wets thesleeve, which has coefficient of expansion higher than that of saidsleeve, and which has a thermal conductivity of at least 0.5, wherebysaid sleeve is contracted away from said insulator nose bore wall by notmore than about 0.001

inch as said material solidies and cools in said split metallic sleeve.

References Cited UNITED STATES PATENTS Cipriani et al. 313-136 XR 8Schwartzwalder et al.

29-25.12 Bretsch et al 313-136 Bychinsky 29-25.12 XR Gregory et al.29-25.12 XR Lever 29-25.l2 XR JOHN F. CAMPBELL, Primary Examiner RICHARDBERNARD LAZARUS, Assistant Examiner

